Orlando, Florida (CNN) -- A whale trainer at SeaWorld died from "multiple traumatic injuries and drowning" after a 12,000-pound killer whale grabbed her ponytail and pulled her underwater in front of shocked onlookers at Shamu Stadium, the Orange County Sheriff's office said Thursday.

Dawn Brancheau, 40, was "pulled underwater for an extended period of time," by the whale, Chuck Tompkins, SeaWorld's curator of zoological operations, told CNN's "American Morning."

The county medical examiner ruled Brancheau "most likely died from multiple traumatic injuries and drowning after one of the park's killer whales pulled her into a pool behind Shamu Stadium," the sheriff's office said in a statement.

The statement confirms Tompkins' account, saying that Brancheau was interacting with the whale, named Tilikum, in knee-deep water "when the animal grabbed her by the hair, said to be in a long ponytail, and pulled her underwater."

Rescuers were not immediately able to reach Brancheau because of the "whale's aggressive nature," the sheriff's office said. She was recovered by SeaWorld staff members after Tilikum was coaxed into a smaller pool and lifted out of the water by a large platform on the bottom of the smaller tank, authorities said.

"While this incident remains the subject of an ongoing death investigation, there are no signs of foul play," the sheriff's statement said. "All evidence and witness statements indicate that the death was a tragic accident."

Earlier accounts varied on how Brancheau ended up in the tank.

A witness told CNN affiliate WKMG-TV that the whale approached the glass side of the 35-foot-deep tank at Shamu Stadium, jumped up and grabbed Brancheau by her waist, shaking her so violently that her shoe came off. A SeaWorld employee, who asked not to be identified, described the incident the same way.

Orange County Sheriff's Office spokesman Jim Solomons said Brancheau slipped into the tank.

Tilikum has been linked to two other deaths. He and two other whales were involved in the drowning of a trainer at a Victoria, British Columbia, marine park in 1991. The trainer fell into the whale tank at the Sea Land Marine Park Victoria and was dragged underwater as park visitors watched.

In 1999, Tilikum was blamed for the death of a 27-year-old man whose body was found floating in a tank at SeaWorld, the apparent victim of a whale's "horseplay," authorities said then.

The Orange County Sheriff's Office said the man apparently hid in the park until after it closed, then climbed into the tank.

The 22-foot-long whale was "not accustomed to people being in his tank" and "wouldn't have realized he was dealing with a very fragile human being," Solomons said at the time.

Because of Tilikum's history, as well as his size, trainers did not get into the water with him, Tompkins told CNN. Specific procedures were in place for working with him, he said, although "obviously, we need to evaluate those protocols."

"He's just a really, really large animal," Tompkins said, noting that female killer whales weigh 6,000 pounds -- half of Tilikum's weight. "Just because of his size alone, it would be dangerous to get in the water with him." But the whale's previous incidents were also taken into account, he said.

Tompkins pointed out that the 1991 incident occurred before SeaWorld owned Tilikum and that no one is sure what took place in the incident eight years later.

Tilikum could have been trying to play with Brancheau or get her attention or companionship, said Nancy Black, a marine biologist who has studied whales for 20 years. Such whales play with seals and sea lions in the wild, tossing them in the air, she said. But they do not kill them and end up letting them go.

"I don't believe the killer whale purposely intended to kill the woman," she said. "It was more likely an accident, I would guess." But, she said, the whale could also have been frustrated for some reason.

Tompkins said there were no indications of any problem with Tilikum or any other animal just before the incident, and that Brancheau "had done a great session with him ... he seemed to enjoy what he was doing at the time."

The incident, however, raises larger questions regarding the captivity of wild animals.

A spokesman for People for the Ethical Treatment of Animals called the death "a tragedy that didn't have to happen."

Jaime Zalac said the organization had called on SeaWorld "to stop confining oceangoing mammals to an area that to them is like the size of a bathtub, and we have also been asking the park to stop forcing the animals to perform silly tricks over and over again. It's not surprising when these huge, smart animals lash out."

Black told CNN that killer whales in the wild live in family groups, and males stay with their mothers their entire lives. Family members rely on each other for social structure and play, and they cover hundreds of miles of ocean, she said.

"I think they do need more space, and situations like that do cause a lot of stress for them, most likely." She said Tilikum had a "flopped fin," something seen in captivity but not much in the wild.

But Tompkins said, "We have a tremendous track record with these animals at SeaWorld" and a very small percentage of problems. It's useful to have animals in the park, he said, because it gives scientists a chance to study them and gives members of the public an opportunity to see them and learn about them.

"This is the first time in 46 years that we've ever had an incident like this with a trainer," he said. Although Tilikum is large and has to be handled carefully, "to mark him as a killer is unfair."

In 2006, a trainer at the adventure park was hospitalized after a killer whale grabbed him and twice held him underwater during a show at Shamu Stadium.